Could you explain what is a difference between readable vs legible?
Which should I use when I want to say "This diagram is easily readable/legible" while meaning - "It is very easy to read and understand meaning of this diagram"?
Dictionaries usually give these as synonyms, but there is a nuance: legible tends to refer only to the presentation, e.g. penmanship, while readable is broader. If what I'm looking at is a hard-to-read scrawl, that's illegible; if it's nicely typed but the grammar and punctuation are all wrong, such that it's hard for me to understand, that's unreadable.
Both legible and readable have sense of "clear enough to read".
Her handwriting was clearly legible.
The figures should be clearly readable
But, readable also may mean "easy, interesting and enjoyable to read".
Legibility is about how easy it is to distinguish individual elements such as letters.
Readability is about how easily blocks of elements—such as paragraphs—are understood.
— Source 1
— Source 2 provides a picture of less-legible, more-readable text, and the opposite.
— Source 3
In the case of diagrams the same idea holds. Illegible could be due to smudges or mis-printed type. Unreadable could be due to a flowchart where the organisation of the elements is confusing.
If you cannot understand what is written though it is clearly written/typed it is incomprehensible (as in you cannot understand it). Legible means that you can read it and illegible means that it is so badly written/scribbled that one cannot read the words at all. Those are two very different situations.